1. Field of the Invention
The present invention relates to abrasive materials and their manufacture and, more particularly, to abrasive particulate and its dispersal in a metal matrix to provide an article of use having superior abrasive properties.
2. Description of the Prior Art
Improved efficiency is an increasingly important factor in the development of gas turbine engines. Such engines have rows of rotating blades fixtured within a generally cylindrical case and leakage of gas between the ends of the blades and case contributes toward inefficiency. In current engine design, this leakage is minimized by blade and sealing systems in which the blade tips rub against a seal which is attached to the interior of the engine case. Generally, it has been found most desirable to construct the system such that the blade tips are harder and more abrasive than the seal and thus cut into the seal during certain stages of engine operation to establish essentially a zero clearance therebetween. Previous abrasive blade tips have been constructed of various materials including certain nickel base superalloys, U.S. Pat. No. 2,994,605, and nickel matrices having tungsten carbide particles randomly dispersed therein, U.S. Pat. No. 3,199,836. However, as a result of the increasing temperatures encountered in gas turbine engines, especially in the turbine section, and of the presence of corrosive gases, the abrading and cutting effectiveness of these blades tips has not been entirely satisfactory. The prior art blade tips have proved especially deficient when used in conjunction with ceramic seal materials which have been developed for gas turbine engine use, for example, graded ceramic seals based on zirconia.
Silicon carbide, being a high temperature hard refractory material would appear particularly useful as an abrasive if incorporated in a metal binder. But the inclusion of silicon carbide, by itself, in metal matrices at elevated temperatures can be calculated and observed to result in interaction with superalloy matrix metals at the temperatures of 2000.degree. F. or higher. Silicon carbide is not sufficiently thermodynamically stable in contact with superalloys. And with the interaction, the hardness and desired cutting properties of the silicon carbide will be lost.
Silicon carbide particles have been included in metal matrices, such as in coated abrasives described in Fontanella U.S. Pat. No. 3,508,890. There, particles positioned on an organic backing are coated with aluminum; optionally, the surface of the aluminum is converted to an oxide, as this is said to aid polishing.
Fisk et al U.S. Pat. No. 3,779,726 describes a method of making metal-abrasive tools containing silicon carbide or other grits. The method comprises encapsulating the grit in a porous metal coating and then impregnating the encapsulating layer with a metal to unite the particles.
However, these prior inventions, being oriented toward abrasives useful at temperatures far below those of gas turbines, do not comprise or teach ways of providing materials which are useful at high temperatures.